This invention relates to glazing panels and particularly but not exclusively to vehicle windscreens provided with electrically heatable coating stacks.
Whilst the primary role of a vehicle windscreen is to permit good visibility for a driver, various additional features may be incorporated into its design. Sensors or emitters arranged inside the vehicle may rely on electromagnetic data transmission through the windscreen. For example, passage of an electromagnetic data signal for automatic payment at the toll barriers used on the French motorway system may pass through the windscreen. It is also known to provide a window in the band of black enamel around the periphery of the windscreen, usually along the bottom edge of the windscreen, through which a vehicle identification number or chassis number, often in the form of a bar code, can be read from the outside of the vehicle.
U.S. Pat. No. 4,668,270 (Ford Motor Company) describes a car windscreen having an electrically heatable coating layer used for defrosting, de-iceing and/or de-misting. The heatable coating, which is laminated between the two glass sheets of the windscreen, is supplied with electrical power via first and second bus bars which extend respectively along the top and bottom edges of the windscreen, each bus bar being silk screen printed on the glass in a silver ceramic material. The heatable coating is a multilayer coating consisting of layers of zinc oxide and silver formed by magnetron sputtering.
Coating layers are well know not only to provide an electrically heatable element but also to modify the optical properties of the glass, particularly to reduce the proportion of incident solar energy which is transmitted through the glass whilst allowing passage of sufficient visible light to ensure good visibility. This can reduce overheating of the interior of the vehicle in summer and is commonly achieved by reflection of incident solar radiation in the infra-red portion of the spectrum. EP378917A (Nippon Sheet Glass Co.) discloses such coating layers. The term solar control coating layer as used herein refers to a coating layer which increases the selectivity of the glazing panel i.e. the ratio of the proportion of incident visible radiation transmitted through the glazing to the proportion of incident solar energy transmitted through the glazing. Many solar control coating layers have the intrinsic property of being electrically heatable.
When a solar control coating is provided on a windscreen it is advantageous for the solar control coating to cover the entire light transmitting portion of the windscreen so as to reflect as much of the incident solar energy as possible. A data transmission window in the form of a gap or hole may be provided in a solar reflecting coating layer specifically to allow the passage of electromagnetic waves through that portion of the glazing, for example to a sensor or emitter. One example of this, as referred to above, is to allow passage of an electromagnetic data signal for automatic payment at the toll barriers used on the French motorway system. The principle is nevertheless applicable to allowing passage of any electromagnetic data transmission signal through a glazing panel, particularly using infra-red wavelengths. The term data transmission window as used herein refers to a portion of the surface area of a glazing adapted to permit electromagnetic data transmission therethrough.